1. Field of the Invention
The present invention relates to a jet-propulsion personal watercraft (PWC), and more particularly to an exhaust system of the personal watercraft.
2. Description of the Related Art
In recent years, so-called jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like.
The personal watercraft is equipped with an engine mounted in a space within a body, surrounded by a hull and a deck. A water jet pump is driven by the engine to pressurize and accelerate water sucked from a water intake generally provided on a bottom hull surface and eject it rearward from an outlet port. Thereby, the personal watercraft is propelled.
In the jet-propulsion personal watercraft, a steering nozzle is provided behind the outlet port of the water jet pump. By operating a bar-type steering handle either to the right or to the left, the steering nozzle is swung either to the right or to the left, to change the ejection direction of the water to the right or to the left, thereby turning the watercraft to the right or to the left.
A line at which the hull and the deck are connected over the entire perimeter thereof is called a gunnel line. The gunnel line is located slightly above a waterline of the watercraft.
In the above personal watercraft, an exhaust gas from the engine is discharged outside the watercraft through an exhaust pipe via which an exhaust port of the engine and an exhaust outlet provided on the body communicate with each other. A muffler (exhaust chamber) is provided at a location in the exhaust pipe to muffle or improve an exhaust noise of the exhaust gas. In order to further enhance a muffling effect of the exhaust noise of the exhaust gas, the exhaust outlet is typically located below the waterline of the watercraft to allow the exhaust gas to be discharged into the water.
The water jet pump is provided at a rear portion of the hull such that the water jet pump is disposed on a center axis of the body along the longitudinal direction of the body. The water jet pump is surrounded by the hull. Conventionally, the exhaust pipe and the exhaust chamber are disposed in a space within the body either on the right or left side of the water jet pump. In recent years, to improve a muffling function and a weight balance of the watercraft, there has been adopted an exhaust system in which the exhaust chamber is divided into two parts in a flow path of the exhaust gas, i.e., an upstream chamber and a downstream chamber arranged on the right and left sides of the water jet pump.
Conventionally, the exhaust pipe, located downstream of the exhaust chamber in the flow path of the exhaust gas, extends substantially in the shape of a straight line from a rear end portion of the exhaust chamber to the exhaust outlet. As disclosed in Japanese Patent Publication No. 3290037, the exhaust outlet of the body, the exhaust pipe, and a portion where the exhaust pipe and the exhaust chamber are connected to each other, are typically located below the waterline of the watercraft.
In the conventional exhaust system having the above configuration, water outside the watercraft enters through the exhaust outlet and easily flows into the exhaust chamber through the exhaust pipe with the engine being in a stopping state without flow of the exhaust gas.
The water flowing into the exhaust chamber sometimes flows into an inside of an engine body during the stopping state of the engine. The water remaining within the exhaust chamber causes a pressure (back pressure) of the exhaust gas to tend to increase when the exhaust gas is discharged during start of the engine. Such a condition is undesirable to the engine.